Historically, most elevated liquid storage tanks in the USA have been constructed entirely of mild carbon steel. During the past ten years, the concept of "composite" elevated tanks has been introduced within the USA. A composite elevated tank utilizes both steel and reinforced concrete. The storage tank (the liquid containing boundaries) are generally constructed of steel and the supporting structure is constructed of reinforced concrete. This method generally utilizes each construction material to its principal advantage; steel as a liquid-tight membrane in tension and reinforced concrete as a supporting structure in compression. This method has been limited to sitecast concrete construction using formwork assembled onsite. Slipformed and jumpformed techniques have been developed for sitecast construction. Details and methods of construction are disclosed in U.S. Pat. Nos. 4,403,460, 4,578,921, 4,312,167, 4,327,531, 4,486,989 and 4,660,336.
Sitecast concrete construction requires a continuous, uninterrupted supply of concrete once a pour has been committed and started. Any delays in delivery of concrete to the jobsite or placement into the forms may lead to coldjoints and/or voids or other defects within the pour. Severe weather (such as heavy rain and/or high winds) which render continued onsite activities impractical or unsafe can result in problems. Temperature extremes, either hot or cold, can have a deleterious effect on the structural and aesthetic qualities of the concrete. Mechanical breakdowns of pumping or placing equipment can cause delays in pouring the concrete into the formwork at higher elevations. Sitecast concrete construction also generally requires a site location that is accessible by concrete trucks from a local ready mix concrete plant.
Examples of precast concrete panels use in the construction of structures such as silos are found in U.S. Pat. Nos. 4,324,081 and 4,555,883. The precast concrete panels of these patents are designed to withstand hoop stresses resulting from containment of solids, semiliquids or liquids within the structures built from the panels. They are not designed to withstand substantial vertical loading as is required of a pedestal used to support a large containment structurs such as a large water tank. Consequently, the panels of these patents emphasize reinforcement in the hoop direction and interconnections at the vertical joints of the structure constructed from the panel designed for strength in the hoop direction without significant attention being paid to design features to withstand large continuous vertical loads coupled with side loads resulting from wind forces and other transverse loads which may be created by the forces of nature.
Reference is also here made to U.S. Pat. No. 3,483,704 which is concerned with a precast concrete panel designed for use as a tunnel liner. Here the design effort is directed at producing a panel which may be used to construct tunnelling capable of withstanding significant pressures, from the outside, which produce compressive hoop stresses. As with the panels designed for silo construction there is little need for significant strength parallel to the axis of structures built from the panels and no design emphasis is placed on strength in that direction.